I'd think that if there were a leak, at least in the unit, that you'd notice it. As the oil
in them is slippery as ****.
Assuming that it's a unit without a locking mechanisim. Then it'd make sense to check, whether, when you pump it up that high, if perhaps the headstay or backstay is stretching in that the frame that you mention. Thus, later, causing the lower pressure reading in your adjuster as the rigging
Simply attach a stick/batten, with measured off markings, to the base of the unit. And add a piece of tape to your backstay, down low, near the unit, to use as your index marker.
Then pump up the unit to X psi, & denote where the piece of tape on your backstay is vs. your measuring batten. And then check it every few hours, or at the end of each watch.
If the tape's moved much in 24hrs, then odds are there's a small leak in the unit somewhere. And if not, then you're rigging's just stretchng a bit, due to the pressure from the backstay adjuster
One other possible cause for the pressure drop, is that when you crank up a backstay, you can literally bend the hull
to curve into a bananna like shape. Either right away, or over a bit of time. Which again, results in the pressure in the backstay adjuster dropping.
And there are tests which you can do for this as well.
Regardless, you can always contact a reputable rigger about it, & or Navtec.
Also, on some units, it's possible to install a mechanical lock onto the unit, so that when you have a lot of load on your backstay for a while, the lock takes the load, instead of the "plumbing". Though I don't think that such is the case on this one, but it couldn't hurt to check. And such a mod might be a non-OEM one.
BTW, if the above description doesn't adequately describe how to test things/make sense, LMK. As I'm low on eloquence right now, due to a caffeine deficency
: What size wire/rod are your headstay & backstay? As most rigging's not hard to stretch significantly to begin with, & if it's thin, then it's that much easier.